1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for generating seismic waves and, more particularly, to mechanisms and techniques for generating seismic waves with buried volumetric sources.
2. Discussion of the Background
Permanent monitoring of oil production (e.g., reservoir monitoring) can be accomplished by passive listening or by active sourcing. Passive listening involves listening with seismic receivers for naturally-occurring seismic waves while active sourcing involves deploying seismic sources over a certain area and generating the seismic waves in a controlled manner. Active sourcing may use sources provided at the surface of the earth or sources buried beneath the surface of the earth. Benefits have been demonstrated from buried sources compared to surface sources. In particular, it has been found that sources buried beneath the weathered layer are more repeatable than surface sources primarily because they are located in consolidated material in a stable coupling environment and not subject to changes due, for example, to soil moisture content that a surface source might see. For time-lapse reservoir monitoring a time-invariant source signal is most helpful.
Conventional land seismic generation generally employs multiple vibrators acting together and initiated simultaneously or randomly. In land-based operations, the vibrators are positioned at a source location and synchronized to a pilot sweep signal. Once activated, the vibrators generate a sweep that typically lasts between five and twenty seconds and typically spans a predetermined range of frequencies. A recording system that is connected to a plurality of receivers, typically geophones for land-based seismic exploration, is employed to receive and record the response data. The vibrators are then moved to a new source location and the process is repeated.
However, existing land sources that are appropriate for burying, e.g., piezoelectric stack vibrators, tend to be relatively low-energy, particularly at low frequencies. Also piezoelectric stack vibrators that undergo axial extension tend to produce significantly more SV-wave energy than P-wave. Thus, a more energetic buried source that also produces more P-wave energy would be advantageous. Therefore, there is a need to develop a land source and associated method that are capable of generating more energy underground than the existing sources.